Hydraulic design of waterways and associated hydraulic structures are necessary to safely and efficiently convey stormwater runoff generated from main roads and highways and to provide crossings of these roads over creeks, floodways and rivers. Hydraulic procedures are required to compute flow characteristics in closed conduits and open channels during the passage of a runoff hydrograph. In all cases, hydraulic calculations are aimed at computing energy losses in the flow conveyance system such that the hydraulic grade line (HGL) of the system can be established. The HGL is often used to determine the location of the hydrostatic head along the water conveyance system. In the case of open channel flow, the HGL generally coincides with the water surface. In closed conduit flow conveyance systems, flow conditions can be pressurised and water rises to the HGL level at junction pits. The design process is commonly a series of iterations involving modifying the various hydraulic structures such that the resulting HGL complies with the design standards specified. These design standards can include:
 Frequency of surcharge of underground drainage systems;
 Flood levels and flow velocities along the river and floodplain;
 Flood attenuation caused by hydrograph characteristics (a function of the hydrograph shape);
 Flood attenuation caused by channel and floodplain storage (a function of the river and floodplain geometrical properties and hydraulic roughness);
 Flow directions, flood breakouts and returns, offchannel storage, etc.
In closed conduit flow conditions, calculation of energy losses within the flow conveyance system is most commonly based on applying energy loss factors to the velocity head (v2/2g). The energy loss attributed to pipe friction is best calculated by the DarcyWeisbach Equation. Localised energy losses due to junction and inlet pits are computed in a similar manner using standard coefficients derived from extensive laboratory tests of these structures.
The simplest model available for computing flood levels and flow velocities is the use of empirical formulae such as the Chezy Equation and the Manning's Equation. These equations are referred to as slopearea methods on the basis that they utilises simple relationships between the discharge in a river to the energy gradient, flow crosssectional area and hydraulic roughness. These formulae are suited for computing water levels only at a single location and for a single discharge value (i.e. steady flow assumptions apply) under uniform flow conditions.
These methods at best would address the second of the above five design considerations related to the hydraulics behaviour of the flow conveyance system. Often the use of slopearea methods are inappropriate due to the fact that natural river flows are not uniform.
Open channel flow conditions can often be categorised as either rapidly varying or gradually varying. Rapidly varied flow occurs whenever there is a abrupt change in the geometry of the channel or in the flow regime of the flow. In regions of rapidly varied flow, the water surface profile changes rapidly. Examples of rapidly varying flow include flow over weirs and through regions of rapid changes in bed elevation or channel width (i.e. abrupt change in geometry) and hydraulic jumps (i.e. change in flow regime). Simulations of rapidly varied flow conditions require the solution to the equations of conservation of mass and conservation of momentum in fluid flow (i.e. the Saint Venant Equations).
The flow condition generally occurring in natural river and floodplain systems can be categorised as that of gradually varied flow; that is, conditions in which the flow characteristics are nonuniform but vary gradually with distance along the channel due to gradual variation of bed slope, channel geometry and hydraulic roughness.
A number of river and floodplain computer models are available which solve either the energy equation (for steady gradually varied flow only) or the Saint Venant Equations either in its complete form or its simplified form. River and floodplain models can be categorised into the following groups:
 lDimensional Steady Flow Models (HECRAS, AFFLUX, and other backwater profile models);
 lDimensional and Quasi 2Dimensional Unsteady Flow Models (MIKE11, DAMBRK, DYNHYD, CELLS, TIDEWAY2D, MIKE12, llDEWAY2DV. etc.);
 2 and 3Dimensional Unsteady Flow Models (SYSTEM21, the TIDEWAY Suite of models etc.).
The applicability of each of the models listed is very much dependent on such factors as:
 data availability;
 resource availability (both in personnel and equipment)
 flow conditions being modelled
The sources of some of the above models are as follows:
HECRAS 
 
Backwater Profile Model developed by the Hydrologic Engineering Center, US Army Corps of Engineers. 
AFFLUX 
 
Backwater Profile Model developed by Main Roads and now available through AUSTROADS. 
MIKE11 & 12 
 
Developed by the Danish Hydraulic Institute and available in Australia from Lawson and Treloar Pty. Ltd. 
DAMBRK 
 
Developed by the US National Weather Service. 
DYNHYD 
 
Developed by the US Environment Protection Agency and available in Australia from Willing and Partners Pty. Ltd. 
CELLS 
 
Developed originally in the University of Witwatersrand in South Africa and modified by various organisations in Australia to improve stability and computation efficiency (ref. Monash University). 
MIKE21 
 
Developed by the Danish Hydraulic Institute and available in Australia from Lawson and Treloar Pty. Ltd. 
TlDEWAY 
 
The TIDEWAY suite of models were developed by Hydraulic Research Limited in Wallingford, UK and are available in Australia from Patterson Britton and Partners Pty. Ltd. The Suite of programs consist of TIDEWAY1D, TIDEWAY2D, TIDEWAY2DV, TIDEWAY3D and TIDEMEAN3D. 
The applicability of each of the models listed is very much dependent on such factors as:
 data availability;
 resource availability (both in personnel and equipment); and
 flow conditions being modelled.
